Rare-earth halides (in what follows, Ln is used to denote a rare earth), especially when they are doped with cerium, and in particular cerium-doped LnBr3 and cerium-doped LnCl3, have very useful scintillation properties especially for applications in nuclear imaging and in spectroscopy (positron emission tomography or PET, gamma camera, oil prospecting and the like). To obtain these properties satisfactorily, it is necessary for these compounds to be obtained in the form of large crystals. Generally, these crystals are single crystals. In certain particular cases, they may be polycrystals, within which the crystals have one dimension of the order of one or more centimeters. However, rare earth halides are highly hygroscopic compounds that react with water and with air as soon as they are heated, forming very stable oxyhalides. It has in general been considered that oxyhalide contents of the order of 0.1% by weight were acceptable, the crystals obtained with these contents being sufficiently transparent in appearance. In addition, certain crystals, such as Csl:Tl, accommodate high oxygen contents (for example around 0.2% of CsOH) as far as the scintillation properties are concerned. Now, the Applicant has discovered that the scintillation properties, especially the luminous efficiency, that is to say the number of UV-visible photons emitted per MeV of energy of an incident particle, of rare-earth halides can be drastically improved by lowering the oxyhalide content in a rare-earth halide crystal below this value.
The Applicant therefore sought to develop manufacturing methods that result in rare-earth halides that are as pure as possible (especially as regards oxygen), that is to say the water content of which is very much less than 0.1% by weight and the oxyhalide content of which is less than 0.2% by weight, and even less than 0.1% by weight or indeed less than 0.05% by weight. Moreover, means have to be found for preserving (for example over several months) and handling these halides that allow this purity to be maintained. This is because the growth of the crystals (generally single crystals) is usually carried out in batch mode, which involves phases of putting them into storage and of removing them from storage, which phases are conducive to contamination of the rare-earth halide by the water and oxygen of air.
In addition, it is very difficult to produce an installation for preparing a rare-earth halide (as raw material for growing crystals, generally single crystals) that does not itself introduce a small quantity of water or oxygen resulting in the formation of an undesirable oxyhalide. This is because any installation is always imperfectly impermeable and also always contains a small quantity of adsorbed water, so that partial contamination is usual in this kind of preparation, and a high degree of oxidation by the impurities of the gaseous environment is generally expected, most particularly at high temperatures such as above 300° C. The invention also provides a solution from this standpoint since the method according to the invention results in a very pure rare-earth halide, even with an installation initially containing water, whether adsorbed, absorbed or in condensed phase, and even in the presence of a reasonable amount of water and oxygen in the atmosphere during the heating leading to melting.